Square Tire setup
 

Is running 235/45/17 all around not enough tire in the back? If I use 7" wheels all around that seems to be the biggest tire that fits the rim. 245/40/17 is better but maybe too big for a 7" rim.

Thoughts?

A 235 will fit on a 7" wheel fine. I never tried a 245.

If you do go square, be careful at first when driving close to the limit in corners. The wider front tires will provide quite a bit of front end bite and when you feel all of that beautiful front end grip, you might be temped to push harder - but beware that the rear will have less grip and at some point, the rear might try to come around in an oversteer condition. It should be fine driving around town but due to the square setup the rear will be more loose at the limit.

In Boxster Spec racing, we run square 255's on 8.5" wheels and use aftermarket adjustable sway bars to adjust the handling from oversteer to neutral.

Using 7x17 I would probably run 235 square. A 235 is oversized for the wheel but it works ok. A 245 is a no go. One of the top drivers in San Diego runs a 225 square on a light weight 2.5 Boxster and he is wicked fast. Close to Mark Foley in a Spec Boxster at Chuckwalla.